At present, photon counting computed tomography (CT) systems having photon counting detectors are known. A photon counting detector outputs a signal capable of individually counting X-ray photons having passed through a subject, unlike an integration detector. A photon counting CT system is thus capable of reconstructing an X-ray CT image having a high signal to noise ratio (SNR).
Signals output from a photon counting detector can be used for measurement (discrimination) of energy of X-ray photons. Thus, a photon counting CT system is capable of dividing projection data, which have been collected by emitting X-rays at one tube voltage, into a plurality of energy components to form an image.
Note that it is essential for measuring X-ray photons having passed through a material and discriminating an object material to calibrate the relation between an output from a radiation detector (a detector output) and the X-ray photon energy incident on the radiation detector (incident energy). Specifically, in a case of what is called an indirect conversion radiation detector, variations in the characteristics (multiplication rate, operating temperatures, etc.) of SiPM elements and variations in scintillation light detection efficiency (variations in detector geometric structure) are caused. Calibration between the output of the detector and incident energy is therefore required. Note that an indirect conversion radiation detector is a radiation detector configured to convert incident X-ray photons into scintillation photons by a scintillator, multiply the scintillation photons by solid silicon photomultiplier elements (SiPM: Silicon Photomultipliers), and output the multiplication result.
In related art, a plurality of checking source (radioactive isotopes) whose energy levels are known are used to identify a peak position in a pulse height distribution for each calibration energy level (a mode in a pulse height distribution), so as to calibrate a detector output and incident energy in association with each other.
In the case of calibration using calibration sources, however, the time required for calibration is determined by the amount of radiation from the calibration sources and the number of available calibration sources. Thus, a radiation detector having an enormous number of pixels such as hundreds of thousands of pixels, for example, has such problems as the number of elements calibrated per unit time being small and the time required for product shipment and apparatus maintenance being long.
In addition, while the calibration work is usually carried out before shipment of radiation detectors (or CT systems), the balance of the output and the incident energy of a calibrated detector may be lost owing to deterioration with time. Since, however, a long time is required for calibration as described above, it is very difficult to stop a CT system for a long time during surgery hours of a hospital to perform calibration again. For a similar reason, it is also difficult to periodically test and calibrate a radiation detector (or a CT system) after being delivered to a hospital.
Furthermore, there are few types of calibration sources supporting energy regions used for calibration. Specifically, while a low energy region of about 50 keV to 120 keV, for example, is used for calibration, calibration sources such as cesium having a long half-life do not support this energy region, and it is therefore difficult to use them as calibration sources. In contrast, in a case of calibration sources where cobalt 57 supporting the aforementioned energy region is used, the half-life is about 271 days, which is very short. Thus, when cobalt 57 is to be used as calibration sources, the calibration sources that are as new as possible need to be always ready in view of the half-life. This is unrealistic in terms of storage area, cost, and the like.